With the steady growth in human population, as well as rapid development of industry and agriculture, access to fresh water is a serious issue. Considering the abundance of seawater on Earth, desalination, which separates dissolved salt ions from a saline feed water source to produce fresh water, is a viable option to achieve an adequate supply of fresh water. Reverse osmosis (RO) is considered an advanced technology for the desalination of seawater. However, RO desalination requires considerable electrical energy input. From the perspective of energy management, capacitive deionization (CDI), which removes salt ions by capturing them in the electrical double layer of high surface area electrodes, is a highly promising alternative, as its operating cost can be reduced significantly compared to the operating cost of RO. However, CDI has limitations in desalinating highly concentrated feed water, such as seawater, because the amount of salt ions that can be stored in the electrical double layer is limited even when high surface area nanoporous carbon electrodes are used. Therefore, CDI has been developed mainly for use in desalination of brackish water.
In order to increase the capacity of an electrode for electrochemical salt removal, salt ions should be stored not just in the double layer but in the bulk of the electrode through the formation of chemical bonds. For example, Pasta et al. constructed a desalination cell by combining MnO2 as the Na-storage electrode and Ag as a Cl-storage electrode, where Na+ and Cl− ions were stored within the electrode structures forming new phases, Na2Mn5O10 and AgCl, respectively. (M. Pasta, C. D. Wessells, Y. Cui, F. L. Mantia, A desalination battery, Nano Lett. 12, 839-843 (2012).) If the desalination cell can store and release salt ions repeatedly through the charging and discharging process, and the discharging process can partially recover the energy consumed during the charging process, such a cell can be considered a rechargeable “desalination battery” and has the potential to achieve desalination with a minimum overall energy input. Considering that the high cost of Ag and the poor electrical conductivity of AgCl limit the use of Ag for practical, large scale desalination cells, the development of desalination batteries depends on the discovery of more efficient, stable, and practical Cl-storage electrodes.